The present invention relates generally to a measuring method, and more particularly to a measuring method that measures the intensity of the light with a target wavelength using a measurement apparatus that can measure the absolute intensity of the incident light, such as a calorimeter. The present invention is suitable, for example, for an exposure apparatus that utilizes the extreme ultraviolet (“EUV”) as a light source.
A conventional reduction projection exposure apparatus uses a projection optical system to project and transfer a circuit pattern of a mask or a reticle onto a wafer, etc., in manufacturing a semiconductor device, such as a semiconductor memory and a logic circuit, in the photolithography technology.
The minimum critical dimension (“CD”) transferable by the reduction projection exposure apparatus or a resolution is proportionate to a wavelength of the exposure light, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Accordingly, use of the exposure light having a shorter wavelength is promoted with recent demands for the finer processing to the semiconductor devices, and ultraviolet (“UV”) light having a smaller wavelength has been used from a KrF excimer laser (with a wavelength of approximately 248 nm) to an ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the UV light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device. Accordingly, a reduction projection exposure apparatus using EUV light with a wavelength of 10 to 15 nm shorter than that of the UV light (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less. The EUV exposure apparatus uses, as a light source, a laser plasma light source and a discharge plasma light source. The laser plasma light source irradiates the laser light onto a target and captures the EUV light from the generated plasma. The discharge plasma light source pinches or converges the plasma generated from the discharge of an electrode under a low-pressure gas atmosphere, and captures the EUV light from the pinched plasma.
Exposure dose control is important for the exposure apparatus, because the transferred pattern's CD depends upon the exposure dose. The exposure dose control relies upon an exposure time period based on the pre-measured exposure dose, or an integral exposure dose based on the exposure dose by taking and measuring part of the exposure light with a photo-detector. The photo-detector that measures the exposure dose is, for example, a photodiode that provides a quick measurement and has high sensitivity. Nevertheless, the photodiode cannot problematically measure an absolute value, or maintain the sensitivity. Hence, a calorimeter that can measure the absolute value is required to regularly calibrate the sensitivity of the photodiode. For this purpose, the exposure apparatus includes a photodiode that always measures the exposure dose, and the calorimeter that calibrates the photodiode.
In measuring the exposure dose in the EUV exposure apparatus, it is proposed to arrange a zirconium (Zr) filter or another filter for shielding the visible light in front of a photodiode and to receive the EUV light reflected on the multilayer coating. See, for example, Japanese Patent Application, Publication No. 2004-303760. The multilayer coating reflects both the far-UV light and the visible light as well as the EUV light having a wavelength near 13.5 nm as the exposure light, and the filter shields both the far-UV light and visible light. While the infrared ray is incident upon the photodiode together with the EUV light from the filter, the photodiode is not sensitive to the infrared ray and can measure the intensity of only the EUV light. The EUV light has photonic energy 100 times as high as the UV light, and the photodiode that measures the EUV light is highly likely to deteriorate.
However, the EUV exposure light cannot use the calorimeter as a detector for calibrating the photodiode or as a detector for measuring the exposure dose, because the calorimeter has the same sensitivity to all lights with various wavelengths and cannot separate the EUV light having a wavelength, for example, near 13.5 nm as the exposure light from other unnecessary lights.
Assume, for example, use of a calorimeter that places a filter in front of it. If only the EUV light is incident upon the calorimeter, the energy of the EUV light at a measured position incident upon the filter is obtained by dividing the measured energy by the transmittance of the filter to the EUV light. However, in fact, the infrared ray emitted from the filter is incident in addition to the EUV light upon the calorimeter. As the filter has such extremely low transmittance to the EUV light that it should be made as thin as possible. For example, even the Zr filter that has high transmittance to the EUV light allows only 50% of the EUV light to pass through it when it is 0.2 μm thick. The absorbed energy cannot escape through heat conduction. In addition, the filter cannot emit the heat to the air since it is placed in vacuum to reduce the absorption of the EUV light in the gas. As a consequence, the energy absorbed in the filter is radiated as the infrared ray and incident upon the calorimeter. The infrared ray from the filter is non-negligible to the EUV light and its intensity amounts to that of the EUV light, causing a significant error, and rendering the calorimeter unusable.
Moreover, the calorimeter is subject to the environmental temperature variance. For example, when the calorimeter is placed in vacuum, the heat incident upon the surrounding components cannot escape, causing the temperature rises in the surrounding members, and the output offset changes. As a result, the output may not become 0 even when the emission of the EUV light stops. The measurement offset varies with time, and an unknown offset amount causes a measurement error.